It is common to have stress fatigue cracks or corrosion damage (collectively referred to herein as "defects") in structural steel components, such as the body of a beam or the wall of a pressure vessel. These defects often eventually require repair.
The conventional technique used involves removing defected metal and replacing it with a compatible alloy through a welding process. The welding can cause undesirable embrittlement, distortion and the imposition of residual stresses, which can lead to further failure.
The research underlying the present invention was undertaken with the aim of devising an alternative technique for repairing such defects, particularly in thick walled steel components.
The research centered on experimenting with carbon fiber-reinforced polymer sheets to form a patch over the defect. More particularly, the commercial product used in the experimentation was one offered by Mitsubishi Chemical Corporation, of Japan, under the trade mark REPELARK.
This technology involves the use of thin, flat, substantially rigid sheets of partly cured epoxy resin having parallel carbon fibers embedded therein and oriented along the length of the sheet. These sheets are referred to in the art by the term "pre-pregs", which is a short-hand term for "pre-impregnated carbon fiber reinforced resin sheet". A liquid epoxy resin (referred to herein as "the curing resin") is used to initiate curing of the resin of the pre-preg and to bond it to the substrate to which it is to be attached. Curing and bonding occur over time when the two resins are brought into contact. Otherwise stated, the Mitsubishi system is characterized by the use of compatible resins which interact chemically at atmospheric temperature to effect complete curing of the pre-preg resin and bonding of the curing resin with the pre-preg resin and the substrate.
This contrasts with other known systems in which carbon fiber-reinforced epoxy sheets are cured using a combination of high temperature (e.g 350.degree. F.) and pressure.
The Mitsubishi system lends itself to use in the field, where it is impractical to use high temperature and pressure-inducing equipment.
In practice, it has been used to clad and reinforce concrete structures, such as bridge beams. In this application, a skin, comprising a single thickness of contiguous pre-pregs, is bonded to the flat concrete surfaces of the beam by a layer of curing resin. The pre-preg resin slowly cures at ambient temperature to a completely hardened state, over a period of about a week, by interaction with the curing resin.
Applicant speculated that the Mitsubishi or a similar pre-preg system might be useful in connection with patching defects in a structural steel component, such as the boom of a crane or the wall of a pressure vessel. However, on carrying out experimentation and obtaining familiarity with the Mitsubishi system, several problems became evident and required solution. More particularly:
The interface strength contributed by the layer of curing resin, when cured at ambient temperature, was very low--in tests, failure would occur in this layer at about 200 N, whereas a failure characteristic in the order of perhaps 10,000 N was needed; PA1 A patch having a thickness of just one pre-preg had insufficient strength in the context of thick-walled steel structures--but using a stack of several pre-pregs required the use of several curing resin layers, which introduced further weakness; PA1 The time required to cure at the conventional ambient temperature in field applications to a fully cured state, (typically a week), was impractical for use with a component that needed to go back into service as soon as possible, fully repaired; and PA1 It was a requirement in many cases that the patch had to conform to a substrate surface that was something other than a flat surface. For example, the surface might be arcuate or irregular (as would occur where a crack extended from a bulging weld). PA1 It was found that mild heating of the pre-preg/curing resin assembly resulted not only in accelerated curing but also in sufficient strengthening of the layer of curing resin so that the latter was no longer a weak point in the end product; PA1 It was further found that use of heat-shrinkable tape was effective to uniformly contain and apply pressure to such an assembly, when mildly heated; PA1 It was found that the provision of wrap-around heating pads and heat-shrinkable tape provided suitable means for applying sufficient pressure and heat to a partly cured pre-preg/curing resin assembly mounted on a steel component in the field to achieve satisfactory bonding and curing within a period in the order of 2 hours; PA1 As a result of the foregoing, it was now possible to use a stack of pre-pregs (perhaps three or more), separated by layers of curing resin, to create an assembly suitable for heat and pressure treatment to produce an end product having the necessary patch strength; PA1 The stack could be satisfactorily contained by a combination of release film and peel-ply fabric (referred to singly or collectively as "release means") at the top and bottom; PA1 To cope with forming problems and to minimize time consumption in the field, advantage was taken of the fact that, in the early stages of curing, the resin of the pre-preg softens and the pre-preg/curing resin assembly can be formed. As curing continues over time, the softened resins harden and begin to rigidify. Therefore, the process preferably incorporates first forming the pre-preg/curing resin laminate stack, including release means, on a prepared form or mould, under controlled conditions at a site removed from the component and then, after about 3 days of curing, conveying the formed, partly cured and shape-retaining laminate stack to the component, mounting it thereon, and rapidly completing curing using mild heat and pressure as previously described; PA1 It was further found desirable to abrade or roughen and clean the surface of the component and to texture the bottom of the stack, after removal of the release means, to ensure good bonding; and PA1 Finally it was found desirable to use a high strength structural adhesive to unite the base of the stack with the surface of the steel component. The bond strength of this layer is the most important parameter in transferring load from the steel substrate to the patch. PA1 The patch/component composite structure had a greater failure strength than the original unreinforced component; PA1 The final steps of applying, bonding and completely curing the stack on the component could be accomplished in about 2 hours; and PA1 Additional crack initiation or growth was effectively arrested.